What's a good mnemonic for shell double vs. single quotes?

Question

I don't do enough scripting to remember, without looking up, whether double or single quotes result in a Unix variable being substituted.

I definitely understand what is going on. My question is does anyone have a memory trick for making the correct quoting rule stick in my head?

Use rc as your shell, then you have only one type of quotes (" is an ordinary character in rc where the only quotes are single quotes and backslash is only used for line-continuation (not inside single quotes)). It's also got a much leaner, cleaner, better, more intuitive syntax and design than Korn-like shells like bash. — Stéphane Chazelas, Oct 26 '17 at 08:45
Learn Perl. Then you just have to memorize that shell behaves similar. — n0rd, Oct 27 '17 at 16:06

score 76 · Accepted Answer · edited Oct 26 '17 at 08:34

76

Single quotes are simple quotes, with a single standard: every character is literal.
Double quotes have a double standard: some characters are literal, others are still interpreted unless there's a backslash before them.

Single quotes work alone: backslash inside single quotes is not special.
Double quotes pair up with backslash: backslash inside double quotes makes the next character non-special.

edited Oct 26 '17 at 08:34

diginoise

103

answered Oct 25 '17 at 19:55

Gilles 'SO- stop being evil'

829,060

7

This is quite poetic. With a good editor (in the literary sense), we could make a children's book out of this. "If You Teach a Snake to Script" – glenn jackman Oct 25 '17 at 20:21
7

I've been trying to turn Single quotes are simple quotes, with a single standard into a Dr. Seuss style poem for twenty minutes, and failing miserably. – Ti Strga Oct 25 '17 at 21:58
7

Single quotes are single use, and lit'rally repeat / Double quotes are more verbose, a paramat'rising feat – DopeGhoti Oct 26 '17 at 16:40
@glennjackman A FORTRAN colouring book – Muzer Oct 27 '17 at 13:18

score 7 · Answer 2 · answered Oct 25 '17 at 19:54

7

Double-quotes (") are weak quotes- you need two of them, so they come in pairs, and variables punch right through them despite their numbers.

Single-quotes (') are strong quotes- they come singly, and variables are no match for them.

answered Oct 25 '17 at 19:54

DopeGhoti

76,081

score 6 · Answer 3 · answered Oct 26 '17 at 20:57

6

"Double, double toil and trouble;
Dollars burn, and backquotes bubble."

Single, in contrast, is harmless.

answered Oct 26 '17 at 20:57

gboffi · Answer 4 · 2021-11-19T07:53:34.657

3

Double quotes, double fun!
Parameters are expanded
One by one.

edited Nov 19 '21 at 07:53

answered Oct 26 '17 at 10:31

gboffi

1,394

If you write 'Params expanded one by one' it fixes the meter. – P i Oct 18 '22 at 10:12

score 1 · Answer 5 · answered Oct 26 '17 at 09:34

1

Double-quotes (") don't exactly mean what the say, for example a tool that says "quality" equipment is probably the opposite.

Single-quotes(') are strong quotes, what is between them is exactly what it seems.

answered Oct 26 '17 at 09:34

Pelle

401

B Layer · Answer 6 · 2017-11-20T14:07:02.457

1

Singular in purpose are they
Quotations that are single.

Treat literal the chars within
No matter how they mingle.

(T'is just one small rule you must know
Single quote within? No go!)

There are those quotes that are double
Inside which some chars may be trouble
Dollar, back-ticks and back-slash
Are treated special by Bash
Escaping, though, will burst that bubble.

edited Nov 20 '17 at 14:07

answered Nov 19 '17 at 20:13

B Layer

5,171

score 1 · Answer 7 · answered Nov 20 '17 at 02:17

1

One: winter, as is
Two: summer, transformations
Forbid or allow

answered Nov 20 '17 at 02:17

drl

838

What's a good mnemonic for shell double vs. single quotes?

7 Answers7

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